The present invention relates to a biaxially oriented polyester film formed by simultaneous biaxial orientation. More specifically, it relates to a biaxially oriented polyester film formed by simultaneous biaxial orientation, which has excellent printing performance without ink transfer nonuniformity even when printing is carried out at a high speed and excellent productivity without a break and delamination during the process or use, and is suitable for use as a base film for a heat-sensitive transfer ribbon.
A film having a specific surface roughness (JP-A 62-299389) (the term xe2x80x9cJP-Axe2x80x9d as used herein means an xe2x80x9cunexamined published Japanese patent applicationxe2x80x9d) is known as a base film for use with a thermal transfer printer. Demand for a subliming transfer recording system out of transfer recording systems is growing because it can output a high-quality full-color image easily. As for this subliming heat-sensitive transfer, a thermally subliming dye is contained in a binder so that only the dye is sublimated by heat and absorbed into the image receiving layer of a transferred sheet to form an image having gradation. In this subliming transfer recording system, a high printing speed is now desired. To increase the printing speed, the method of transmitting heat from a thermal head at the time of printing efficiently is effective and, accordingly, a reduction in the thickness of a base film is desired. However, when the thickness of a conventional stretched film is simply reduced to meet this demand, the following problem arises. For example, when a dye ink is applied to a thin film or in the slitting step, work efficiency lowers due to a reduction in the thickness of a film.
This work efficiency is related to the slipperiness of a film. To improve this, fine irregularities are generally formed on the surface of the film. More specifically, there are known a method (external particle addition method) in which inert particles are added during or after the polymerization of a thermoplastic polymer which is a raw material of a film, a method (internal particle precipitation method) in which part or all of a catalyst used for the polymerization of a thermoplastic polymer is precipitated in the polymer in the reaction step, and the like.
However, in the method of producing an extremely thin film, when a thin film is formed from a polyester containing inert particles in the same amount as a normal film, the number of inert particles per unit area decreases, the interval between adjacent inert particles on the surface of the film expands, and the surface of the film becomes too flat, thereby reducing slipperiness. Therefore, in order to compensate for deterioration in slipperiness caused by a reduction in the thickness of the film, the amount of inert particles to be contained or the diameter of the particles must be increased as the thickness of the film decreases.
In this case, a large number of voids are often produced at the interface, that is, around the inert particles due to low affinity between the inert particles and the thermoplastic polymer at the time of melt extrusion or stretching at a high draft ratio. As the result of the production of voids, fine particles easily fall off from the obtained film, chippings produced by contact between the film and a roller contaminate the process, the film easily breaks, productivity lowers, and production conditions lack stability. Particularly when a coarse particle is existent on the slit section, the film breaks from that particle.
To solve the above problems, JP-A 2000-103874 proposes that 0.1 to 2 wt % of calcium carbonate having an average particle diameter of 0.5 to 4 xcexcm and 0.05 to 1 wt % of aluminum silicate having an average particle diameter of 0.1 to 2.0 xcexcm are contained in a biaxially oriented polyester film obtained by sequential biaxial orientation. According to the publication, since affinity between the inert particles and the thermoplastic polymer is improved even at the time of melt extrusion or stretching at a high draft ratio, voids produced at the interface therebetween, that is, around the inert particles can be suppressed.
According to researches conducted by the inventors of the present invention, the problems that the process is contaminated and that the film is easily broken by chippings produced by contact between the film and the roller have been overcome due to above biaxially oriented polyester film which voids are supressed to be produced. However, a new problem has been found that when the film is used as a subliming heat-sensitive transfer material, a ribbon is deformed by the heat of a thermal head, thereby reducing the reproducibility of an image.
Heretofore, means of improving dimensional stability has been employed to suppress the deformation of the ribbon. More specifically, the molecular chain is more highly aligned by stretching the film at a higher draft ratio or more highly crystallized by thermal-setting the film at a higher temperature or for a longer time. However, when the molecular chain is highly oriented or highly crystallized by sequential biaxial orientation to solve this problem, delamination often occurs, thereby causing a break and a product defect with the result of reduced productivity.
It is therefore an object of the present invention to provide a biaxially oriented polyester film which solves the above problems, has excellent productivity while ensuring a transferred image having excellent shape reproducibility and gradation and is particularly suitable for use as a base film for a heat-sensitive transfer ribbon.
Other objects and advantages of the present invention will become apparent from the following description.
According to the present invention, the above objects and advantages of the present invention are attained by a biaxially oriented polyester film which (A) contains 0.1 to 2 wt % of first inert inorganic particles having an average particle diameter of 0.5 to 5 xcexcm, (B) satisfies the relationship HS/F5xe2x89xa60.028 in which HS is a heat shrinkage factor (%) in the longitudinal direction of the film when it is heated at 200xc2x0 C. for 10 minutes and F5 is an F-5 value (N/mm2) in the longitudinal direction of the film at 23xc2x0 C., and (C) is formed by simultaneous biaxial orientation.